A piston and cylinder unit is known from German patent number DE 101 56 504 C1 corresponding to U.S. Pat. No. 6,837,144 B2. The known piston and cylinder unit includes a cylinder housing, a piston including an opening and a piston rod having a common axis. The piston includes a piston seal and a guiding element. The piston with its inner thread being located in the opening is connected to the outer thread of a protrusion of the piston rod without an increased tightening moment. The inner thread of the piston and the outer thread of the piston rod are arranged to have the same eccentricity with respect to the respective axes of the piston and of the piston rod. The piston is screwed on the piston rod at such an angle end position that the axes of the piston and of the piston rod are aligned. The eccentricity is used to prevent unintended rotational movement without requiring additional securing elements. This means that the fixed arrangement of the piston advantageously only requires a few components and thus can be produced at low costs. It is not even necessary to arrange engagement surfaces for a rotational tool at the piston since the piston can be screwed in both directions by hand. Increased tightening moments are not used such that it is also not necessary to harden the faces of the piston and of the piston rod. The known piston connection has a short structural length and thus makes it possible that the full length of the piston rod can be used.
Furthermore, it is also known in the art to fixedly connect a piston in a piston and cylinder unit, the piston having an opening and being designed to be continuous in an axial direction and not to include a thread. The piston is pushed over a protrusion of the piston rod. The protrusion of the piston rod includes an outer thread onto which a securing nut is screwed after having slipped on the piston. The securing nut has an opening and an associated inner thread. For example, the securing nut includes an insert made of plastic serving as securing element against a rotational movement in the loosening direction. In this way, a non-positive securing effect against undesired rotation is attained.
It is also known in the art to combine a piston and a securing nut to one single part such that the associated inner thread is arranged in an opening of the piston. A stud being arranged in the piston in a radial direction is used as a securing element, the stud in the mounted position pressing against the protrusion of the piston rod. The known piston has to include a respective engagement surface for a rotational tool to be capable of applying the required high tightening moment during assembly and the respective high loosening moment during disassembly. Usually, it is necessary to use special tools for this purpose.
Both above described known arrangements for fixing a piston using a high tightening moment have the following drawbacks: It is not possible to apply the high moments during assembly and disassembly without special tools. The piston or an additional securing element has to include respective engagement surfaces for the special tool. Additional components for preventing unintended rotational movement are required, and they lead to the structural length in the region of the piston being increased. The front surfaces of the piston and of the piston rod substantially extend in a radial direction. They contact each other and thus need to be hardened in a comparatively complicated way. Such piston connections are complicated in manufacture, and they are not easy to be serviced.
In case it is required to arrange a static seal between the piston and the piston rod, the piston and the piston rod are designed and arranged such that there is an overlap in an axial direction to an extent required for arranging the static seal. In this way, the surface of the piston rod, which has anyway been accurately processed, maybe used as the counter surface for the static seal.
Another piston and cylinder unit is known from U.S. Pat. No. 3,293,993. The known piston and cylinder unit includes a cylinder housing, a piston having an axis and a piston rod having an axis. The piston includes an opening including an inner thread with which it is connected to an outer thread being located on a protrusion of the piston rod. The piston is screwed onto the thread of the protrusion of the piston rod with a high tightening moment such that there are high pulling forces in this part of the piston rod. The axes of the threads and the axes of the piston rod and of the piston coincide. The known piston and cylinder unit does not teach any means for preventing rotational movement of the piston with respect to the piston rod, and it also does not disclose a design which is safe and easy to be serviced.
A piston arrangement for a fluid motor is known from German patent application number DE 24 48 019 corresponding to U.S. Pat. No. 3,885,461. The known piston and cylinder unit includes a cylinder housing, a piston including an opening and a piston rod. The piston includes an opening including an inner thread which is screwed onto and tightened to an outer thread of a protrusion of the piston rod with a high tightening moment such that an edge of the piston “bites” into a diminishing section of the piston rod. A locking nut is located on another section of the protrusion in a way that it contacts the piston under pressure. The threads of the locking nut and of the additional section of the protrusion of the piston rod are less declined and have a greater number of threads than the threads of the piston and of the section of the piston rod being connected thereto. All threads are designed to be rotated in the same direction. In this way, rotational forces during operation which usually lead to loosening effects now act in a way that the locking nut and the piston are tightened even closer in a direction towards one another.
To design the arrangement of the piston in a way to make it easier to be serviced, another piston and cylinder unit is known in the art. The piston and the piston rod are loosely screwed on each other by a threaded connection including an inner thread and an outer thread. It is important in this known connection that any rotation of the piston with respect to the piston rod is prevented. The connection is attained without applying a tightening moment. The means for preventing rotation is designed as a bolt, a bush and the like, which protrudes into an axial bore. The bore is arranged in a way that it protrudes through the threaded portion between the piston rod and the piston. Chips are produced during assembly. These chips result in undesired soiling. The bush, the bolt or the like additionally has to be secured against an axial movement. Usually, this is achieved by a screw including a securing disk, the screw protruding through the piston in an axial direction and which forms a stop for the movement of the bush, the bolt or the like. This known fixed arrangement of the piston includes a comparatively great number of components serving to prevent rotational movement. Due to the design and arrangement of the elements, the structural length in the region of the connection of the piston and of the piston rod is comparatively great. Fixing the piston is complicated and causes high costs due to the need for manufacturing these elements. However, it is advantageous in this known piston arrangement that no high tightening moment is to be used such that the piston connection is easy to be repaired since, after having removed the securing means, the piston can be loosened by a comparatively easy rotation of the piston rod.